Why Buy Belled End?
Labor is a significant contributor to overall manufacturing costs involving welded components, and careful consideration for the design of pipe systems for plants or processes can save significant time and labor, while maintaining – or increasing – system performance. Selecting components that enable strong, quickly accomplished welds yields great returns.
Increasingly, belled end pipe fittings are the choice in a variety of demanding applications in which forged socket, cast socket, and formed butt weld fittings were typically used. Cold wrought belled end fittings have tangent ends shaped into a “bell” profile, creating a socket that can be fillet welded. This eliminates the need for butt-welded joints.
Belled end pipe fittings have been around for a while, favored for providing a strong, easily produced and inspected welded joint. Driven by the Navy’s need for more reliable and cost effective welds in piping on surface ships, belled end pipe fittings became standardized nationally in 1996 under MSS SP-119. Presently, belled end pipe fittings are widely used in the chemical, petrochemical, pulp and paper, power generation, and pharmaceuticals industries, as well as in most Navy ships and submarines.
Belled end welded fittings provide several benefits over butt-welded fittings. Butt-welded joints take four to seven times as long to produce as fillet welded joints because of the need for joint preparation: When butt welding, both pipe pieces must be prepared (beveled) on site for a precise fit. Belled end fittings require no machining of either piece on site before welding.
Pre-weld fit time with belled end fittings is nearly zero, compared with the significant amount of time it takes to fit butt-welded joints. Small misalignments between fitting and pipe can make butt welds difficult or impossible to achieve. With belled end fittings, alignment and wall thickness matching are much less critical: they are far more forgiving of deviations in thickness, alignment, or out-of-round than standard fittings. Belled end socket clearances will accommodate small misalignments and are allowed by Piping Codes, with no degradation in pipe joint performance. All this makes for a very fitter/welderfriendly piping system.
Typically it takes only one pass to create a good fillet weld. Butt-welded joints, by contrast, often require multiple weld passes, as well as a backing ring to create a greater surface area for material deposition at the joint between the pipe and fitting. Joints that are butt-welded without a backing ring must first be clamped into a jig and tack welded. Either approach requires additional welds to fill the v-shaped groove created by the joining of the beveled ends.
Fillet welding a belled end pipe joint requires less labor and skill than creating a butt-welded joint, increasing the number of welds that can be achieved in a work shift.
Reducing misaligned or mismatched welds improves overall system reliability. Because it is easier to produce a high quality fillet weld than it is a properly executed butt-welded joint, the percentage of successful welds increases with belled end pipe fittings.
Besides taking more actual welding time, the multiple welding passes needed with butt welding must also be visually inspected. The fillet weld used to join belled end fittings to pipe can be easily visually inspected. Moreover, if there is an error in pipe piece fabrication, the belled end fitting can easily be cut out and reused.
Tests carried out by the U.S. Navy demonstrated that belled end fittings meeting MSS SP-119 matched the burst capacity of ASME B16.9 rated butt-welded fittings.
Belled end fittings joined to pipe with fillet welds become stronger than the pipe, alone. The fillet weld creates a high quality, solid joint. Belled end pipe fittings will handle pressures to 1000 psi and temperatures of 200 degrees Fahrenheit.
In situations where fatigue is a concern, the belled end fittings actually performed better. One reason is that cold wrought belled end fittings have wall thicknesses very close to the pipes they are joining. Cast or forged fittings tend to be rigid and oversized. In thin wall piping, this often creates a “notch” effect at the joint and leads to a weakness in fatigue. By contrast, when welded into the process line, the close match of the belled end fittings and pipe creates a flex continuum, moving stresses away from the joint and into the sidewalls of the fittings. In applications where pipe flexing and vibrations are an issue, belled end fittings can deliver a longer life cycle.